Generally, one of contents included in the standard of the third generation wireless mobile communication system is adaptive modulation and coding (AMC) scheme. For this, 3GPP TR 25.848 V4.0.0 (2001-03) “3rd Generation Partnership Project; Physical layer aspects of UTRA High Speed Downlink Packet Access” (available on http://www.3gpp.org) can be referred to.
The AMC scheme is the scheme for dynamically changing modulation and coding scheme (MCS) according to a channel status. In generally, a receiver observes a channel status to select suitable MCS and then feeds back the selected MCS to a transmitter. According to the AMC scheme, a variation of channel quality due to multi-path fading or user's movement can be compensated for to some extent.
One of general criteria used in determining MCS is to estimate a channel quality. By estimating a channel quality, an optimal MCS enabling a data rate to be maximized under the restriction of a target QoS (quality of service) is selected. Generally, a signal-to-noise ratio (Hereinafter abbreviated SNR) is frequently used as a channel quality. For example of the SNR estimation, David R. Pauluzzi, Norman C. Beaulieu, A comparison of SNR estimation techniques for the AWGN channel, IEEE Trans. on Comm., vol. 48, no. 10, pp. 1681-1691, October 2000 can be referred to.
To raise efficiency of AMC scheme, an accurate estimation of channel quality and a stable feedback channel path are needed. So, the accurate estimation of the channel quality is necessary for the efficient AMC scheme.
In general, a time delay is present between channel information of a receiver and that of a transmitter. Yet, in a channel environment having relatively large user mobility, a channel path currently estimated by the receiver may differ from a channel path which will be transmitted by the transmitter due to the time delay. For instance, a user currently located in an open space may enter a crowded space between buildings.
So, to raise performance of the AMC scheme, a time delay of channel information needs to be compensated for in a channel environment having user mobility.
Meanwhile, schemes (AMC included) for enhancing system performance through feedback normally provide maximum performance in case of receiving feedback information including a channel status and a user's moving speed, and the like from a receiving end at every time the data is transmitted. Yet, if feedback information is transmitted on every transmission, a feedback channel is overloaded. In particular, it is unable to effectively distribute channel resources in a multiple access system.
To solve this problem, a scheme for performing feedback according to a preset cycle without considering a channel status has been proposed by a related art.
However, this scheme has to calculate and transmit feedback information with a predetermined cycle even if a channel or a user's moving speed is not considerably changed. So, unnecessary loads are given to a receiving end a feedback channel.
Moreover, even if a channel or user's moving speed is considerably changed, the related art scheme is unable to receive feedback information unless a prescribed cycle arrives. So, the previous feedback information should be used to cause system degradation.
So, a method of effectively reducing a feedback information size smaller than that of the related art by maintaining throughput of a transmitting end is requested.